This invention relates to a caster angle control apparatus and method for a suspension of a vehicle.
Heretofore, a suspension apparatus for a vehicle was known which comprised an actuator for varying an arm length and the mounting position on the vehicle body of a specific suspension arm, and a controller for controlling the actuator. When the controller drove the actuator, relative positions of the arms and struts of the suspension apparatus were varied, which varied the alignment of the suspension apparatus, that is, a caster angle and a trail of the suspension apparatus, a toe-in angle and a camber angle of the vehicle wheels, and the like. The controller actively operated the alignment according to the traveling condition of the vehicle to improve the straightforward running stability, turning stability, and the like of the vehicle.
The caster angle is very important in view of the steering characteristics and running stability. However, when the caster angle is increased, when the vehicle wheels deviate from the straightforward running position during traveling, a righting moment to return the wheels to the straightforward running position is increased to improve the running stability during high-speed traveling. However, operation of the steering wheel becomes heavy during low-speed traveling, resulting in deteriorated steering characteristics. On the contrary, when the caster angle is decreased, operation of the steering wheel during low-speed traveling becomes light, but the running stability at a high speed will be degraded.
Then, for example, Japanese Patent Laid-open Publication (OPI) 59-67111 proposed an alignment control method, in which the caster angle was variable in response to the vehicle speed, that is, the caster angle was increased during high-speed traveling, and decreased during medium- or low-speed traveling. By varying the caster angle according to the vehicle speed, ease of steering during medium- or low-speed traveling, and a steering response during high-speed traveling were obtained, thereby improving the steering characteristics and the high-speed running stability.
However, although such an alignment control method provided ease of steering during medium- and low-speed traveling and a steering response during high-speed traveling, stability (convergence) of the vehicle tended to be deteriorated during operation of the steering wheel, particularly during rapid steering, in a transitional condition, or when the frictional condition of the road surface changed.
Then, Japanese OPI 4-87884 proposed an alignment control method controlled the caster angle in consideration of a steering angle or a steering angular velocity and the road surface condition. However, since, in such an alignment control method, the caster angle was increased according to an increase in the steering angle or the steering angular velocity in all conditions, stability of the vehicle could have possibly been impaired. Further, in the above alignment control method, the road surface condition meant an external disturbance factor, and it was not always a control according to a change in the frictional condition of the road surface. On the other hand, Japanese OPI 5-131951 proposed an alignment control method which increased the caster angle as the frictional coefficient of the road surface decreased. However, such an alignment control method intends to notify the driver of a normal steering angle limit due to a decrease in the frictional coefficient of the road surface, but was quite different from one which stabilized the vehicle against changes in frictional condition of the road surface, and thus did not always improve the stability (convergence) of the vehicle.